The present invention relates to a distortion compensation circuit for use in an amplifying device; and, more particularly, to a distortion compensation circuit for reducing an unbalance between a higher 3rd order distortion and a lower 3rd order distortion generated by an amplifier of the amplifying device.
In general, a distortion is generated in the course of amplifying a signal in an amplifier. Therefore, in a communications device which amplifies a communications signal by using an amplifier, it is needed to cancel a distortion caused in the course of amplifying a signal, e.g., a W-CDMA (wide-banded code division multiple access) signal or a multi-carrier signal, by using the amplifier.
FIG. 1 illustrates, as an amplifying device using a distortion compensation method of the prior art, an example of an amplifying device which cancels a distortion by using a distortion compensation circuit adopting a feed-forward method.
In the amplifying device shown in FIG. 1, an input signal (a main signal) is distributed into two ways at a distributor 41. A distributed signal is amplified by an amplifier (a main amplifier) 42 to thereby be inputted into a subtractor 44, and another distributed signal is inputted into the subtractor 44 through a delay line 43. In the subtractor 44, a distortion component is extracted by deducting the signal inputted from the delay line 43 from the amplified signal inputted from the main amplifier 42. The distortion component is inputted therefrom into a distortion amplifier 45 and the amplified signal, which includes the distortion component, inputted from the main amplifier 42 is outputted to another subtractor 47 through another delay line 46. Further, the distortion component, which is extracted at the subtractor 44, is amplified at the distortion amplifier 45 to thereby be outputted to the subtractor 47. The subtractor 47 generates a final amplified signal without a distortion by deducting the amplified distortion component inputted from the distortion amplifier 45 from the amplified signal, which includes the distortion component, inputted from the delay line 46.
Here, the signal inputted from the delay line 46 to the subtractor 47 is generated by amplifying the input signal in the main amplifier 42 and the signal includes a distortion caused by the main amplifier 42. Further, the signal inputted from the distortion amplifier 45 to the subtractor 47 is generated by amplifying the distortion. Therefore, the output signal from the subtractor 47 is considered to be a signal generated by canceling the distortion caused by the main amplifier 42, i.e., deducting the distortion from the amplified signal generated by the main amplifier 42. Further, each of the distributor 41 and the subtractors 44 and 47 comprises, e.g., a directional coupler.
However, in such an amplifying device as described above, the efficiency in the main amplifier 42 is known to be very poor, since an amplified signal outputted from the main amplifier 42 is attenuated while passing through the subtractor 44, the delay line 46 or subtractor 47, which requires an increase in an output level of the main amplifier 42 in accordance with a required output level of the amplifying device.
Meanwhile, FIG. 2 exhibits an example of an amplifying device having a distortion compensation circuit adopting a pre-distortion method.
The amplifying device of FIG. 2 has a pre-distortion circuit 51 coupled to an input of a main amplifier 52. The pre-distortion circuit 51 generates a pre-distortion before a main signal being generated, a phase of the pre-distortion having a difference of 180xc2x0 with respect to a distortion (i.e., an opposite phase) and a same amplitude as that of the distortion, the distortion being generated by the main amplifier 52. Therefore, the distortion caused by the main amplifier 52 is canceled by the pre-distortion generated by the pre-distortion circuit 51.
Such an amplifying device can be implemented to obtain a high efficiency since any other circuits are not coupled to an output of the main amplifier 52. However, in this case, the distortion generated by the pre-distortion circuit 51 should have same characteristic as that generated by the main amplifier 52 regardless of the variation or frequency characteristics of an input signal.
Here, it is understood by those skilled in the art that the distortion caused by the main amplifier is due to an AM-AM (amplitude modulation-amplitude modulation) conversion or an AM-PM (amplitude modulation-phase modulation) conversion.
FIG. 3A charts a graph showing an example of the AM-AM conversion performed in the main amplifier. The horizontal and vertical axes of the graph represent an input level and a gain of the main amplifier, respectively. FIG. 3A shows an ideal gain characteristic (G1) together with those (G2 and G3) of the main amplifier and the pre-distortion circuit. As shown in FIG. 3A, the ideal gain characteristic (G1) can be obtained by combining those (G2 and G3) of the main amplifier and the pre-distortion circuit.
Further, FIG. 3B exhibits a graph showing an example of the AM-PM conversion performed in the main amplifier. The horizontal and vertical axes of the graph represent an input level and an output phase of the main amplifier, respectively. FIG. 3B shows an ideal phase characteristic (P1) together with those (P2 and P3) of the main amplifier and the pre-distortion circuit. As shown in FIG. 3B, the ideal phase characteristic (P1) can be obtained by combining those (P2 and P3) of the main amplifier and the pre-distortion circuit.
Referring to FIG. 2, the underlying principle of the pre-distortion now will be described.
In FIG. 2, xcex1 represents an instantaneous power of a signal inputted to the pre-distortion circuit 51, xcex2 represents an instantaneous power of a signal outputted from the pre-distortion circuit 51 and inputted to the main amplifier 52, and xcex3 represents an instantaneous power of a signal outputted from the main amplifier 52.
An I/O characteristic of the main amplifier 52 can be expressed like as Equation (1) by using xcex2 and xcex3. Here, A is a vector representing a gain and a phase of a small signal generated by the main amplifier 52, B is a vector representing a gain and a phase of a 3rd order distortion generated by the main amplifier 52, and C is a vector representing a gain and a phase of a 5th order distortion generated by the main amplifier 52. Further, each of A, B, C, a, b, and c, which will be described later, is expressed by a vector, i.e., (gain coefficient, phase coefficient).
xe2x80x83xcex3=Axc2x7xcex2+Bxc2x7xcex23+Cxc2x7xcex25xe2x80x83xe2x80x83(1)
Likewise, an I/O characteristic of the pre-distortion circuit 51 can be expressed like as Equation (2) by using xcex1 and xcex2. Here, a is a vector representing a gain and a phase of a small signal generated by the pre-distortion circuit 51, b is a vector representing a gain and a phase of a 3rd order distortion generated by the pre-distortion circuit 51, and c is a vector representing a gain and a phase of a 5th order distortion generated by the pre-distortion circuit 51.
xcex2=axc2x7xcex1+bxc2x7xcex13+cxc2x7xcex15xe2x80x83xe2x80x83(2)
If xcex2 is eliminated by substituting Equation (2) for xcex2 in Equation (1), an equation, i.e., Equation (3), showing a relationship between xcex1 and xcex3 can be obtained.                                                         γ              =                            ⁢                                                A                  ·                  a                  ·                  α                                +                                                      (                                                                  A                        ·                        b                                            +                                              B                        ·                                                  a                          3                                                                                      )                                    ·                                      α                    3                                                  +                                                                                                      ⁢                                                                    (                                                                  A                        ·                        c                                            +                                              3                        ·                        B                        ·                                                  a                          2                                                ·                        b                                            +                                              C                        ·                                                  a                          5                                                                                      )                                    ·                                      α                    5                                                  +                ⋯                                                                        Equation        ⁢                  xe2x80x83                ⁢                  (          3          )                    
In the amplifying device as shown in FIG. 2, a distortion canceling can be accomplished by setting each of coefficients for xcex13 and xcex15 in Equation (3) to zero, which is expressed by using Equations (4) and (5).
Axc2x7b+Bxc2x7a3=0xe2x80x83xe2x80x83(4)
xe2x80x83Axc2x7c+3xc2x7Bxc2x7a2xc2x7b+Cxc2x7a5=0xe2x80x83xe2x80x83Equation (5)
In the pre-distortion circuit 51 as shown in FIG. 2, characteristics satisfying Equations (4) and (5) should be realized. Further, if the conditions expressed in Equations (4) and (5) are satisfied, the amplifying device generates neither 3rd order intermodulation distortion (IM3) nor 5th order intermodulation distortion (IM5).
However, as shown in FIGS. 3A and 3B, characteristics of the AM-AM and the AM-PM conversion are so complicated that the characteristics of the pre-distortion circuit must have a complicated function to implement an amplifying device having the above described ideal characteristics. Therefore, it is so difficult to calculate coefficients of the characteristic functions by using an analog approach.
Accordingly, as an alternative amplifying device having a distortion compensation circuit adopting the pre-distortion method, there has been proposed an amplifying device as shown in FIG. 4.
In the amplifying device of FIG. 4, an input signal, e.g., an RF (radio frequency) signal, is branched by a branch circuit 61. A branch signal is outputted from the branch circuit 61 to a phase circuit 67 through a delay line 62. Another branch signal is outputted from the branch circuit 61 to an amplitude detector (envelope detector) 63.
The amplitude detector 63 detects an amplitude level (envelope level) of the inputted branch signal. Here, a detecting method used by the amplitude detector 63 may be based on, e.g., a square-law detection, but not limited thereto.
And then, the detected amplitude level is converted into a digital signal by an A/D (analog to digital) converter 64. The digital signal is inputted to a table 65a for phase correction and a table 65b for amplitude correction.
The table 65a stores data for the correction of a phase of a signal together with a corresponding amplitude level of the signal. Therefore, when the digitized amplitude level outputted from the A/D converter 64 is inputted to the table 65a, corresponding data for phase correction are read from the table 65a to thereby be outputted to a D/A (digital to analog) converter 66a. The D/A converter 66a converts the data for phase correction into an analog signal, the analog signal being inputted to the phase circuit 67 through an LPF (low pass filter) 73a. 
Meanwhile, the table 65b stores data for the correction of an amplitude of a signal together with a corresponding amplitude level of the signal. Therefore, when the digitized amplitude level outputted from the A/D converter 64 is inputted to the table 65b, corresponding data for amplitude correction are read from the table 65b to thereby be outputted to a D/A converter 66b. The D/A converter 66b converts the data for amplitude correction into an analog signal, the analog signal being inputted to an amplitude circuit 68 through an LPF 73b. 
The branch signal, which is delivered from the branch circuit 61 to the phase circuit 67 through the delay circuit 62, is synchronized with the data for amplitude and phase correction outputted from the D/A converters 66a and 66b through LPF""s 73a and 73b. 
Accordingly, in the phase circuit 67, the delayed branch signal inputted from the delay circuit 62 is distorted in its phase by using the data for phase correction from the D/A converter 66a. And then, in the amplitude circuit 68, the phase-distorted signal is distorted in its amplitude by using the data for amplitude correction from the D/A converter 66b. 
The amplitude and phase distortion imposed on the delayed branch signal by the phase circuit 67 and the amplitude circuit 68 is subsequently cancelled by amplitude and phase distortion generated by a main amplifier 69. That is, the amplifying device has such a characteristic that AM-AM or AM-PM conversion is performed therein based on an input level thereof. However, as shown in FIGS. 3A and 3B, such a characteristic is cancelled by an opposite characteristic which is generated by the amplitude and phase correction data stored in the tables 65a and 65b, which results in an ideal characteristic of the amplifying device.
The signal amplified by the main amplifier 69 is outputted as a final output signal through another branch circuit 70. Further, in the branch circuit 70, a part of the amplified signal from the main amplifier 69 is branched to a distortion detection circuit 71.
The distortion detection circuit 71 extracts a distortion component remaining in the branch signal from the branch circuit 70 after the distortion canceling, the remaining distortion component being outputted to a table update circuit 72.
The table update circuit 72, based on the distortion component detected by the distortion detection circuit 71, calculates amplitude and phase correction data for further canceling the distortion component remaining in the branch signal from the branch circuit 70. Subsequently, the amplitude and phase correction data are stored into the tables 65a and 65b. In this way, the amplitude and phase correction data stored in the tables 65a and 65b are updated to minimize the amplitude and phase distortion caused by the amplifying device. Further, through the update of the amplitude and phase correction data by using the above described feed-back system, the amplifying device can operate in an efficient manner regardless of any effect caused by, e.g., a temperature change or a secular change.
However, the degree of the distortion generated by the main amplifier depends on the frequency of the RF signal, which results in unpredictable characteristics of the distortion.
FIG. 5 shows an example of two output signals and corresponding distortions generated by a main amplifier when two input signals, each of which has a different frequency, e.g., f1 or f2, are inputted to the main amplifier. In FIG. 5, the horizontal and vertical axes represent frequency and amplitude level of the signals, respectively. FIG. 5 charts IM (intermodulation) distortion components, i.e., a lower 3rd order distortion and a higher 3rd order distortion at frequencies of (2xc2x7f1xe2x88x92f2) and (2xc2x7f2xe2x88x92f1), respectively, wherein f2 is larger than f1 (f2 greater than f1).
As shown in FIG. 5, when the amplitude levels of the two input signals are identical, there is introduced an amplitude difference xcex94IM (=Bxe2x88x92A) between an amplitude level A of the lower 3rd order distortion and B that of the higher 3rd order distortion at the frequencies of (2xc2x7f1xe2x88x92f2) and (2xc2x7f2xe2x88x92f1), respectively. In this case, although the pre-distortion circuit of the amplifying device operates in an ideal state, an identical distortion canceling process is performed with respect to the whole range of frequencies, such that a distortion component corresponding to the amplitude difference xcex94IM cannot be canceled by such a distortion canceling process.
Further, such an amplitude difference, xcex94IM, is caused by a distortion factor other than what the main amplifier has in general. For example, ordinary 3rd order distortion components, which are generated by the main amplifier, have same amplitude levels at the frequencies of (2xc2x7f1xe2x88x92f2) and (2xc2x7f2xe2x88x92f1), respectively.
Even when the ordinary distortion components, i.e., 3rd order distortion components, can be cancelled by the pre-distortion circuit having characteristics opposite to those of the 3rd order distortion components, xcex94IM shown in FIG. 5 cannot be cancelled by the pre-distortion circuit. For example, when A, B and xcex94IM are set to be 1.0, 0.8 and 2 dB=0.2, respectively, a distortion component other than the ordinary distortion component becomes 0.1 and the ordinary distortion component becomes {B+(Axe2x88x92B)/2}=0.9. In this case, since the distortion component other than the ordinary distortion component remains after distortion canceling performed by the pre-distortion circuit, the amount of distortion canceling becomes only |20 Log(0.1/0.9)|=19 dB. Further, the larger xcex94IM becomes, the smaller the amount of distortion canceling becomes.
Meanwhile, the amplifying device shown in FIG. 1, which has the distortion compensation circuit adopting the feed-forward method, can obtain an amount of distortion canceling of more than 30 dB. Therefore, as for the amount of distortion canceling, the amplifying device using the feed-forward method operates in a more efficient manner than that using the pre-distortion circuit.
There are several factors that cause the amplitude difference xcex94IM. For example, a distortion having a difference frequency of (f2xe2x88x92f1) is generated due to an even order distortion caused by a transistor included in the main amplifier, and the input signals having frequencies of f1 and f2 are modulated due to the distortion caused by the transistor. This is one of factors that may cause the amplitude difference xcex94IM. Such a distortion becomes more apparent when, as in an AB-class amplifier, the variation of drain current thereof is relatively large. Further, when an input signal having a frequency component of f1 or f2 is mixed with a signal having a frequency twice the frequency component, i.e., 2xc2x7f1 or 2xc2x7f2, the amplitude difference xcex94IM may be also introduced.
In the following, the underlying principle of the distortion canceling performed by the prior art will be described in more detail.
FIG. 6A illustrates spectra of IM distortions and an input signal having frequencies of f1 and f2 after the amplification thereof by the amplifier. In FIG. 6A, the horizontal and vertical axes represent frequency and level of signals, respectively.
FIG. 6B charts lower and higher 3rd order IM distortions, each of which has a frequency of (2xc2x7f1xe2x88x92f2) or (2xc2x7f2xe2x88x92f1) and is represented as a vector a1 or a2, respectively. In FIG. 6B, the horizontal and vertical axes represent frequency and level of signals, respectively. As shown in FIG. 6B, the lower and higher 3rd order distortion components are phase-shifted clockwise by +xcex81 and +xcex82 at the frequencies of (2xc2x7f1xe2x88x92f2) and (2xc2x7f2xe2x88x92f1), respectively, when the AM-PM conversion together with the AM-AM conversion is performed. Further, each of the lower and higher 3rd order distortion components is asymmetric with respect to each other (i.e., they have different level and phase) due to a frequency characteristic of the input signal.
A description on such a phase change as described above is given in Suematsu, Iyama and Ishida, xe2x80x9cTransfer Characteristic of IM3 Relative Phase for a GaAs FET Amplifier,xe2x80x9d IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 45, NO. 12, DECEMBER 1997.
Further, as shown in FIG. 6C, due to a factor other than a common factor for causing distortions, i.e., the AM-AM/AM-PM conversions or the frequency characteristic, another distortion, i.e., a vector b1, is added to the lower 3rd order distortion at the frequency of (2xc2x7f1xe2x88x92f2). Also, another distortion, i.e., a vector b1, is added to the higher 3rd order distortion at the frequency of (2xc2x7f2xe2x88x92f1).
As shown in FIG. 6C, if the two vectors a1 and b1 representing distortions at the frequency of (2xc2x7f1xe2x88x92f2) are summed, the lower 3rd order distortion can be represented as a vector c1=a1+b1. Likewise, the higher 3rd order distortion at the frequency of (2xc2x7f2xe2x88x92f1) can be represented as a vector c2=a2+b2. Here, |c1| less than |c2| and each of the vectors c1 and c2 has a different phase with respect to each other, i.e., +xcex83 or +xcex84 clockwise, respectively, which means that there exists an unbalance between the IM distortions.
FIG. 6D describes results of distortion canceling performed on the distortions shown in FIG. 6C by using the distortion compensation circuit as shown in FIG. 4. As shown in FIG. 6D, the distortion compensation circuit of the prior art can cancel the distortion, which is represented as the vector c1, by generating a negative vector (xe2x88x92c1) with respect to the vector c1. However, a distortion represented as a vector d=c2xe2x88x92c1, which is phase-shifted by +xcex85 clockwise, remains after the distortion canceling is performed on the distortion at the frequency of (2xc2x7f2xe2x88x92f1).
As described above, the amplifying device of the prior art, having the pre-distortion circuit for distortion canceling, has such a problem that there is an unbalance, i.e., an amplitude and/or phase difference, between the higher 3rd order distortion and the lower 3rd order distortion, wherein the unbalance cannot be canceled by the pre-distortion circuit.
It is, therefore, an object of the present invention to provide a distortion compensation circuit for reducing an unbalance between a higher 3rd order distortion and a lower 3rd order distortion generated when an input signal having more than two frequency components is amplified by an amplifier.
In accordance with a preferred embodiment of the present invention, there is provided a distortion compensation circuit for reducing an unbalance between a higher 3rd order distortion and a lower 3rd order distortion generated by an amplifier which amplifies an input signal having at least two frequency components, the distortion compensation circuit including: amplitude modulation means for performing an amplitude modulation on the input signal by using a control signal having a frequency corresponding to a difference between the frequency components wherein the distortion compensation circuit cancels the unbalance by using sideband signals generated as a result of the amplitude modulation performed by the amplitude modulation means.
Here, the amplitude modulation means may include control signal generating means for generating a control signal based on an envelope of the input signal; amplitude adjusting means for adjusting an amplitude of the control signal; phase adjusting means for adjusting a phase of the control signal; and amplitude modulation performing means for performing an amplitude modulation on the input signal based on the control signal to thereby generate the sideband signals.
Further, the distortion compensation circuit may further includes: distortion level detecting means for detecting levels of higher and lower 3rd order distortions included in the input signal after the distortion canceling being performed; and sideband signal adjusting means for adjusting the sideband signals generated by the amplitude modulation means to thereby reduce a difference between the levels.
In accordance with another preferred embodiment of the present invention, there is provided an amplifying device adopting a pre-distortion canceling method, the amplifying device including: a distortion compensation circuit in accordance with the preferred embodiment of the present invention.
Here, the amplifying device may further includes distortion level detecting means for detecting levels of higher and lower 3rd order distortions included in the input signal after the distortion canceling being performed; and sideband signal adjusting means for adjusting sideband signals generated by the amplitude modulation means to thereby reduce a difference between the levels.
In accordance with still another preferred embodiment of the present invention, there is provided a distortion compensation circuit for reducing an unbalance between a higher 3rd order distortion and a lower 3rd order distortion generated by an amplifier which amplifies an input signal having at least two frequency components, the distortion compensation circuit including: amplitude modulation means for performing an amplitude modulation on the input signal by using a control signal having a frequency corresponding to a difference between the frequency components; and phase modulation means for performing a phase modulation on the input signal by using a control signal having a frequency corresponding to a difference between the frequency components wherein the distortion compensation circuit cancels the unbalance by using sideband signals generated as a result of the amplitude and phase modulations performed by the amplitude modulation means and the phase modulation means.
Here, the amplitude modulation means may include control signal generating means for generating a control signal based on an envelope of the input signal; amplitude adjusting means for adjusting an amplitude of the control signal; phase adjusting means for adjusting a phase of the control signal; and amplitude modulation performing means for performing an amplitude modulation on the input signal based on the control signal to thereby generate the sideband signals.
Further, the phase modulation means may include control signal generating means for generating a control signal based on an envelope of the input signal; amplitude adjusting means for adjusting an amplitude of the control signal; phase adjusting means for adjusting a phase of the control signal; and phase modulation performing means for performing a phase modulation on the input signal based on the control signal to thereby generate the sideband signals.
Further, the distortion compensation circuit may further include distortion level detecting means for detecting levels of higher and lower 3rd order distortions included in the input signal after the distortion canceling being performed; and sideband signal adjusting means for adjusting the sideband signals generated by the amplitude modulation means and the phase modulation means to thereby reduce a difference between the levels.
In accordance with still another preferred embodiment of the present invention, there is provided an amplifying device adopting a pre-distortion canceling method, the amplifying device including: a distortion compensation circuit in accordance with the preferred embodiment of the present invention.
Here, the amplifying device of claim may further include distortion level detecting means for detecting levels of higher and lower 3rd order distortions included in the input signal after the distortion canceling being performed; and sideband signal adjusting means for adjusting the sideband signals generated by the amplitude modulation means and the phase modulation means to thereby reduce a difference between the levels.